Gas chromatography is essentially a physical method of separation in which constituents of a vapor sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
In some applications, a fluid source, such as a carrier gas supply and/or a sampling device, such as a headspace sampler or thermal desorption unit, is connected to the chromatographic column via a transfer line. This transfer line, which may, for example, comprise a length of fused silica tubing, communicates the fluid from the source to the column for separation and detection. In certain applications, an additional device may also be provided for performing some additional pre-concentration of analytes, such as in the system disclosed in U.S. Pat. No. 6,652,625 to Tipler, the contents of which are herein incorporated by reference in their entirety.
In some applications, as the column is heated, the viscosity of the gas flowing through it likewise increases. As a result, under isobaric conditions—where the carrier gas is applied at a constant pressure—the flow rate through the column will decrease as the temperature of the column increases. Though this usually has no detrimental effect on system performance in some applications, in other applications, such as those that employ a flow-sensitive detector, such as a mass spectrometer, the effect on performance can be dramatic.
The viscosity varies with respect to changes in temperature in a relatively predictable manner for the common carrier gases—a relationship that can be approximated according to the equation:
                              η          c                =                                            η              0                        ⁡                          (                                                T                  c                                                  T                  0                                            )                                x                                    (        1        )            where: ηc is the viscosity at column temperature Tc                 η0 is the viscosity at absolute temperature T0 (from published tables)        x is a dimensionless constantThe coefficients for the three most common carrier gases, for example, are provided in the following table:        
TABLE 1GasT0 (K)η0 (Pa · s × 10−6)xHydrogen273.28.3990.680Nitrogen273.216.7360.725Helium273.218.6620.646
Accordingly, by determining the column temperature Tc, one can determine the viscosity ηc using Equation 1 and Table 1.
When the viscosity nc is determined, presuming the column dimensions are known, a specific flow rate can be entered and maintained using the Hagen-Poiseuille equation as follows:
                              F          o                =                              π            ·                          d              c              4                        ·                          (                                                P                  i                  2                                -                                  P                  o                  2                                            )                                            256            ·                          L              c                        ·                          η              c                        ·                          P              o                                                          (        2        )            Where: Fo is the flow rate at the column outlet                dc is the internal diameter of the column        Lc is the length of the column        ηc is the viscosity of the carrier gas in the column        Pi is the carrier gas pressure at the column inlet        Po is the carrier gas pressure at the column outlet        
Some gas chromatographs are equipped with electronic programmable pneumatic controls. Therefore, because the relationship between viscosity and temperature is well known as described above, and because the GC oven temperature is known due to the fact that it is controlled by the same system, the chromatograph is able to readily compensate for the above-described changes in gas viscosity by increasing the column inlet pressure at a rate calculated to maintain a constant (isochoric) flow rate through the column.
In some applications, however, the gas pressure is controlled on a device remote from the chromatograph, such as a sampling device. This requires that the sampling device have constant knowledge of the column temperature in order to calculate the viscosity at that temperate and make the appropriate adjustments to the applied pressure.
Accordingly, another solution that has been proposed is to monitor the temperature of the column, as is disclosed in U.S. Patent Application No. 2006/0016245 by Tipler et al, the contents of which are herein incorporated by reference in their entirety. In such systems, a temperature sensor may be employed to measure the temperature of the column and communicate this measurement to the sampling device, and the sampling device then adjusts the pressure at which it supplies the fluid based, in part, upon this temperature.
In order to effect the above-described pressure compensatory approach, the sampling device must know the geometry and temperature of both the transfer line and the column, unless the pressure is controlled at an interface between the two. In some cases, an interface device is employed to control the flow rate of the fluid flowing into the chromatographic column. For example, in U.S. Patent Application No. 2005/0284209 by Tipler et al, the contents of which are herein incorporated by reference in their entirety, a system is disclosed in which a chromatographic injector interfaces a transfer line with the column, and this injector is used to control the flow rate at the column inlet.